Not applicable.
Not applicable.
The invention relates to a holder for flat workpieces, particularly semiconductor wafers.
The miniaturization of semiconductor components which has steadily intensified over the recent years causes more stringent and new demands to the manufacturing process of the electronic components. Thus, the surface of the semiconductor material to be exposed during the lithographic printing process has to be very flat (the difference in profile being less than 0.4 xcexcm) if the structure sizes are less then 0.5 xcexcm in order to lie within the focussing plane. To this effect, the material requires to be planarized by means of suitable devices.
A process serving the purpose is the chemico-mechanical polishing method (briefly called CMP). In this process which uses a polishing agent which is both corrosive and abrasive, the wafer is polished on a polishing cloth in plastic at a defined contact force under a rotatory motion of the polishing cloth and the wafer. While the polishing process is under way the polishing agent (a slurry) will flow onto the polishing cloth and form a film between the cloth and the wafer. The slurry which is used consists of a chemically offensive solution to which particles such as silica are added in a colloidal suspension.
From DE 195 44 328 or the company document xe2x80x9cCMP Plaster Tool System Planarization Chemical Mechanical Polishingxe2x80x9d published by the Wolters GmbH company in March, 1996, it has been known to provide appropriate stations and devices for such polishing processes. The wafers are retained by holders in processing units and are pressed by them against the polishing working surface. The holders or holding heads are connected to a spindle of a driving machine which is supported to be adjustable in height in order to press the wafer against the working surface. To obtain sufficient planarity, the lower support plate which holds the wafer via vacuum channels or bores is hinged by a universal joint to a carrier portion which, in turn, is connected to the spindle of the driving mechanism. The contact pressure is applied to the support plate via the universal joint.
From DE 197 55 975 A1, it further has become known to guide a support plate for the known holder in a carrier so as to be movable in height and to dispose an annularly closed membrane between the carrier portion and the support plate. The enclosed inner space of the membrane is optionally connected to the atmosphere or a vacuum or a fluid source under pressure. The pressure and vacuum help in displacing the support plate relative to the carrier. In this way, the contact pressure is applied to the support plate on a large surface, which causes an improved result to be obtained in planarization.
Apart from influencing other parameters such as the speed of the wafer, the speed of the polishing disk, the oscillating motions of the polishing head, the supply of polishing agent, and the condition and wear of the polishing cloths, the accuracy and uniformity which can be achieved will have an effect on the result of polishing in the CMP process. Planarized films of 300 mm wafers which are processed by CMP machines frequently present a rotationally symmetric, differentiated surface geometry which is characterized in that the wafer border is heavily polished, the removal of material is least at a small distance from the wafer border, i.e. 3 mm, and the largest removal of material is achieved in a range of abt. 20 mm from the wafer border.
It is the object of the invention to provide a holder for flat workpieces, particularly semiconductor wafers, in which the non-uniformity of the remaining film thickness is reduced.
The inventive holder provides a ring-shaped loading member of limited width which is supported to be movable parallel to the axis in the support plate near the border and is displaceable by a loading device towards a workpiece retained by the support plate and away from the device to exert a predetermined pressure on the workpiece.
The ring-shaped loading member which is brought very close to the border of the support plate, e.g. to a distance of about 3 mm, and which only extends over a limited width, e.g. from 5 to 10 mm, helps in generating a separate extra pressure if a pressure is exerted on the workpiece, particularly the wafer, by means of the support plate. Such a measure allows to equalize the removal of material across the overall area of the workpiece, particularly the wafer, to a larger extent.
According to an aspect of the invention, restoring means are provided which displace the loading member in a direction away from the contact surface of the support plate if the loading mechanism is turned off. This ensures that if the workpiece is received by means of a vacuum in order to make the workpiece bear on the support the loading member does not interfere therewith.
Various possible ways are imaginable to form a loading member and to actuate it. For an actuation, it is preferred that a pneumatic pressure be employed all the more so as it is known and advantageous to produce a contact pressure with the support plate via a fluid pressure. It is particularly advantageous to use a ring-shaped hose which is accommodated in a ring-shaped recess of the support plate. The hose, which is preferably elongate in cross-section, may be expanded by means of a gaseous medium and, thus, can exert a pressure on the workpiece. If the material of the hose yields resiliently the hose may be restored automatically if it is relieved from pressure.
The fluid pressure in the hose-shaped loading member is preferably controlled via a proportional-pressure control valve. This allows to apply a finely proportioned pressure in a purposeful way.
The invention has the advantage that it may be installed in conventional holders. Thus, for example, it is unnecessary to continue employing the retaining ring, which is normally used and which bears against the polishing cloth, in the form which is known.